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UBC Theses and Dissertations

The significance of some insular characteristics in birds Grant, Peter R.


The reported tendency for island birds to possess larger wings and bills than their mainland counterparts, was investigated by a study of the passerine birds of the Tres Marias Islands, Mexico. It was found that several species showed this bill trend, but that a tendency for birds to possess larger wings was less apparent; an unexpected tendency for the tarsus of island birds to be large was also demonstrated. On The other hand, there is no apparent tendency for the body-size of island birds to be larger than that of their mainland counterparts. The bill and tarsus characteristics of the Tres Marias birds are also shown by many other passerine birds in islands situations elsewhere in Mexico and North America. The differences in dimensions between mainland and Tres Marias birds cannot be explained as adaptations to different climatic conditions (Allen’s rule). Observations of feeding behaviour and analysis of gizzard contents of collected specimens reveal that some island species have a diet which is different from that of their mainland counterparts and that their use of perches is different too. This suggests that the bill and tarsus are used differently in the two regions. It is postulated that large bill and tarsus are adaptations to a greater exploitation of the environment. Differences inhabit at between the mainland and islands do not account for differences in feeding behaviour, but the relative paucity of specie on the islands offers and explanation. It suggests that there is some more environment available for exploitation to each of those present on the islands that to those on the mainland, assuming that there are approximately equal resources in the two regions. There is additional indirect evidence to suggest that the absence of several species has important effects upon those present. Island birds tend to be more drab in plumage that mainland birds on the Tres Marias and elsewhere and at least one species on the Tres Marias has reduced vocal repertoire. In view of the value of distinctive plumage and song as specific recognition characteristics the futures displayed by the Tres Marias birds may be attributed to the absence of systematically closely-related species. At least one species nests considerably lower in the vegetation On the Tres Marias Islands than on the mainland, which may be due to the relative absence of predators on the islands. There is evidence to suggest that the ecological conditions which have permitted the evolution of bill and tarsus characteristics in island birds, have permitted the evolution of large body-size in island rodents also. If this is true, then it is possible that the recorded tendency for animals to become larger in the course of evolution is related to an increase in the amount of environment available for exploitation.

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